Swelling in O-rings: Cause and Protection
When an O-ring swells, it takes up volume and loses strength. This is how swelling develops, how to recognise it, and how to select the right material and groove.
- During swelling, the medium penetrates the O-ring, the volume increases and the strength decreases.
- Even slight swelling can lead to failure in dynamic applications, because friction increases.
- Fluorinated elastomers such as FKM and FFKM (ECOLAST®) barely swell in mineral-oil-based media.
- If the O-ring swells too much, there is a risk of groove overfill. Design the groove fill level with a reserve.
- Know the medium, concentration and temperature, then choose the suitable material.
What is swelling in O-rings?
Ideally, O-rings perform their task completely unobtrusively. If they fail, high downtime costs quickly follow, or entire product batches have to be discarded because of possible contamination. One of the most common causes is swelling.
Swelling is a physical influence on the O-ring caused by the process medium. The medium penetrates the elastomer material, and the O-ring increases in volume. Unlike chemical attack, the material initially remains elastic but loses part of its strength.
In operation, O-rings come into contact not only with air, water and lubricants. Even these supposedly harmless substances can attack elastomer materials. In addition, industry uses numerous aggressive process media. Selecting the right O-ring material for the specific case, with the appropriate media resistance, is decisive for a durable seal.
How the medium causes swelling
In industrial use, O-rings encounter a wide variety of media. In the food industry these include oils and flavourings, along with aggressive cleaning and sterilisation media. In the pharmaceutical and chemical industries, O-rings meet the full range of acids, alkalis and solvents.
Damage caused by process media can follow two mechanisms. Physical influence usually leads to swelling: the medium penetrates the elastomer material. The result is an increase in volume that can damage the polymer matrix.
Chemical attack, by contrast, directly attacks the network of long-chain polymers that gives O-ring materials their elastic properties. The old long-chain bonds are replaced by shorter segments, or the material is cross-linked further. This makes the material hard and brittle. Read more in the overview of the causes of failure in O-rings.
Consequences of the volume increase
Through swelling, the sealing element loses part of its strength but remains elastic. As a result, it is more prone to break-outs or gap extrusion.
The reverse effect is also possible. During shrinkage, the contact medium extracts compound constituents from the O-ring. The O-ring loses volume, so that a minimum compression between the sealing surface and the groove base is no longer ensured. The consequence here is likewise the loss of the sealing effect. Both processes can be at least partly reversible.
Slight and severe swelling
How strongly swelling takes effect depends heavily on the application. Especially in dynamic applications, comparatively slight swelling can already lead to failure. The reason: in such systems, friction is particularly decisive.
Expected swelling should therefore be taken into account when designing the installation situation. During failure analysis, experts look, among other things, for pronounced swelling or shrinkage as well as a tackiness of the surface as a sign of media influence.
Material selection against swelling
The basis for the media resistance of an O-ring is created by the base polymer. It also determines further fundamental properties such as ageing behaviour and temperature limits. Added to this are numerous formulation constituents such as fillers, plasticizers, anti-ageing agents or vulcanization accelerators, which tune the properties of the vulcanizate to the requirements.
As a result, when selecting materials, engineers usually have to weigh several factors against each other. An overview of the swelling behaviour of common materials:
| Material | Behaviour under media influence |
|---|---|
| EPDM | Well suited to hot water, but swells with certain greases and oils. Avoid contact with these substances as early as assembly and storage. |
| NBR | Very good resistance to oils and natural gases. Not suitable for hot process water or solvents, limited ageing resistance. |
| VMQ (silicone) | Physiologically inert and good for food applications, but shows weaknesses with the frequently used steam sterilisation. |
| FKM and FFKM (ECOLAST®) | Fluorinated elastomers with good resistance to mineral-oil-based media. They barely swell in these liquids and are well suited to long-term use. |
| PTFE (TEFLON) | Similar resistance profile to FFKM, but without the elasticity that is central to seals. As a solid material, rather unsuitable for mechanical loads. |
As a fully fluorinated rubber, FFKM represents a further development of FKM. It is the elastic O-ring material with the best resistance properties against solvents, aggressive acids and alkalis. However, the costs are comparatively high. PTFE-encapsulated elastomers combine media resistance with the ability to change shape flexibly.
Groove fill level and groove overfill
When an O-ring swells, groove overfill can occur. The volume of the O-ring then significantly exceeds the installation space defined by the width and depth of the groove.
Therefore, the groove fill level should leave a reserve from the outset. Expected swelling belongs in the design of the installation situation from the start, so that the O-ring remains securely in the groove even in the swollen state and is not pushed into the sealing gap.
Remedy and prevention
Given the appropriate experience in handling sealing elements, many causes of failure can be avoided. Three levers above all help against swelling:
In the event of a seal failure, a thorough failure analysis is important. Often media, temperatures and mechanical load interact in complex ways and are difficult to foresee at the time of commissioning.
Frequently Asked Questions
What is swelling in an O-ring?
What triggers swelling?
Which material swells the least?
What happens with groove overfill?
Is swelling reversible?
How do I recognise swelling on a removed O-ring?
